This invention relates to acquisition of control or other information signals which included an identifiable code sequence such as a direct spreading sequence.
It is often necessary to detect, acquire, and track the frequency and time of a received direct-sequence spread-spectrum (DS/SS) signal or of a unique-word message preamble in order to reliably de-spread andor demodulate the received signal. In direct-sequence spread-spectrum, the transmitted information bits are multiplied or spread by a high rate pseudorandom (PN) spreading sequence.
In many DS/SS systems, detection and acquisition of received signal timing and frequency is difficult because the signal energy is well below the associated noise level. In code-division multiple access systems in which multiple users access the same frequency bandwidth by use of different spreading codes having advantageous cross-correlation properties, the noise contribution from the codes of the other users adds to the thermal noise to exacerbate the detection and acquisition problem.
ordinary prior-art approaches to the acquisition problem are based mainly on correlation or matched filtering operations performed on the received signal, variants of which are termed sliding correlation, serial search, and recursion aided sequential estimation. In the presence of a signal, many of these techniques ultimately generate a signal correlation or autocorrelation function, the peak of which should indicate signal timing. However, the noise and other interferences associated with the signal may make determination of the correlation peak difficult. Frequency errors or cumulative phase errors between the information carrier and the local frequency will tend to decrease the magnitude and increase the spread of the peak correlation.
A standard technique for acquisition of signals in the presence of frequency errors or frequency shifts is to use frequency bin searching, either by use of a plurality of correlators operating in parallel, one for each frequency of the received signal, or by searching frequency serially, or both. The use of one correlator for each possible frequency requires a great deal of hardware (or equivalent software), but provides fast acquisition. Serial searching of the frequencies requires only one correlator, but may require significant time to complete a search, especially considering that each attempted correlation may take as long as the recurrence period of the spreading code. Thus, there is a tradeoff between speed and complexity; two correlators could be used in parallel to search using various different frequency hypotheses, thereby presumably cutting the search time in half (by comparison with a single comparator using serial search) by a doubling of the complexity of the equipment.
Correlation or filtering is well known, and can be implemented by many techniques, as for example by Finite Impulse Filters (FIR) or by use of Fast Fourier Transforms (FFT).
Improved techniques for determination of the presence of spread-spectrum signals, or of the frequency andor time of spread-spectrum signals, is desired.
A method according to the invention is for acquisition, which may be in the presence of Doppler shift, of a received message with at least a preamble encoded by a pseudorandom spreading code or other identifiable code sequence. The method includes the step of generating a plurality of different phases of the spreading code. The different phases of the spreading code may be stored in memory for later use, if desired. A plurality of the different phases of the spreading code are summed together or linearly superposed, to thereby produce summed PRN code signals. If desired, the summed PRN code signals may be stored for later use, together with (or instead of) the different phases of the spreading code. The incoming spreading code is correlated with the summed PRN code signals, to thereby generate a cross-correlation signal. The cross-correlation signal indicates whether (or if) the desired phase is among those phases in the summed PRN code signals. If the cross-correlation signal indicates that the desired phase is among those phases in the summed PRN code signals, individual cross-correlations are performed between the incoming spreading code and those of the phases of the spreading code which make up the summed PRN code signals. This identifies the particular phase of the spreading (or other identifiable) code by which the message should be decoded.
A particularly advantageous mode of the method according to the invention includes the further step, after the correlating step, of determining if (a) the cross-correlation signal indicates that the desired phase is among those phases in the summed PRN code signals and (b) the number of different phases in the summed PRN code signals exceeds a particular number, and if both conditions exist, dividing the summed PRN code signals into at least two portions, each having fewer different phases than the particular number, and performing the step of correlating the incoming spreading code with at least one of the portions.